Trans World Airlines Inc. paid part of the $1-million rent it owes Denver International Airport. Denver International received a check for $645,000, an airport spokesman said. The St. Louis-based airline said in a letter accompanying the check that an upgrade of its accounting systems has disrupted some processing of payments. On Thursday, the carrier paid the $1.2 million it owed airports in San Francisco and Dallas. A TWA spokesman declined to discuss why the airline had fallen behind on its payments for landing fees and gate space. TWA had $181.6 million in cash at the end of last year--a low bank balance for a business its size. The carrier's losses widened to $284.8 million last year, up from $227.5 million in 1995.